Constant Gas Density Research Articles

Influence of the pressure and temperature on laser induced breakdown spectroscopy for gas concentration measurements

Laser induced breakdown spectroscopy (LIBS) is an effective technique to obtain quantitative concentration information for the fuel injection and combustion processes. Practical applications of fuel injection and combustion usually involve a wide range of ambient pressures and temperatures, but understanding of the influence of these parameters on LIBS is far from adequate. The objective of this paper is therefore to investigate the pressure and temperature effects on LIBS over a wide range for gas concentration measurements, and further correct these effects to improve the measurement accuracy. The calibrations at various ambient pressures show that the peak intensity ratios (PIRs) exhibit better linear correlations with the equivalence ratios than the area intensity ratios (AIRs) for H656/N746. The intensities of H656 and N746 are remarkably influenced by the gas pressure due to the combined effects of varied gas density and self-absorption. While the PIR of H656/N746 depends greatly on gas pressures from 0.3 to 1.0 bar, the gas pressure effects on the PIR of H656/N746 are very slight at gas pressure above 1.0 bar. The effects of gas temperature on the PIR of H656/N746 are investigated at both constant gas pressure and constant gas density. The results show that the temperature differences between the calibrations and test conditions would significantly degrade the measurement accuracy, especially at low gas pressures. Then, a novel method to correct the gas temperature effects on LIBS for gas concentration measurements is proposed, based on a linear correlation between the PIR of H656/N746 and the gas temperature, with the gas temperature estimated by the full width at half maximum (FWHM) of H656.

Internal gas models and central black hole in 47 Tucanae using millisecond pulsars

Despite considerations of mass loss from stellar evolution suggesting otherwise, the content of gas in globular clusters seems poor and hence its measurement very elusive. One way of constraining the presence of ionized gas in a globular cluster is through its dispersive effects on the radiation of the millisecond pulsars included in the cluster. This effect led Freire et al. in 2001 to the first detection of any kind of gas in a globular cluster in the case of 47 Tucanae. By exploiting the results of 12 additional years of timing, as well as the observation of new millisecond pulsars in 47 Tucanae, we revisited this measurement: we first used the entire set of available timing parameters in order to measure the dynamical properties of the cluster and the three-dimensional position of the pulsars. Then we applied and tested various gas distribution models: assuming a constant gas density, we confirmed the detection of ionized gas with a number density of $n= 0.23\pm 0.05$ cm$^{-3}$, larger than the previous determination (at 2$\sigma$ uncertainty). Models predicting a decreasing density or following the stellar distribution density are highly disfavoured. We are also able to investigate the presence of an intermediate mass black hole in the centre of the cluster, showing that is not required by the available data, with an upper limit for the mass at $\sim 4000$ M$_{\odot}$.

Variable gas density effects on transport from interacting evaporating spherical drops

The paper reports an analytical approach to model heat and mass transfer from neighbouring spherical drops accounting for the dependence of the gas mixture density on temperature and composition. The conservation equations for single-component non-identical spherical drops evaporating in gaseous quiescent environment are analytically solved in a bi-spherical coordinate system. Analytical expressions for the local heat and mass fluxes on drop surfaces and for the evaporation and the sensible heat rates are reported. The effect of relieving the commonly adopted assumption of constant gas density is quantified by comparison with existing constant density solutions for different evaporating species and operating conditions. The model applies to any value of Lewis number. Comparison with analogous results for single isolated drops allows to quantify the screening effect on vapour and heat fluxes and evaporation and heat rates.

Characterization of heat recirculation effects on the stabilization of edge flames in the near-wake of a mixing layer

A fundamental study aimed at investigating the stabilization characteristics of edge flames established in the near-wake of two merging streams, one containing fuel and the other oxidizer, is presented, with the main focus placed on the effects of the thermal interaction between the flame and the splitter plate. To this end, a diffusive-thermal model characterized by constant gas density and transport coefficients is used for conditions at which flame liftoff is likely to occur. It is assumed that the incoming streams are of equal strain rates, that the fuel and oxidizer are supplied in stoichiometric proportion, and that the mass diffusivities of the reactants are equal, such that the resulting combustion field is symmetric with respect to the centerline extending from the splitter plate. The results indicate that the plate has a negligible effect on the edge flame unless the tip of the plate intrudes into the preheat zone of the curved premixed flame segment forming the edge flame. In an overall adiabatic system, the heat conducted from the flame to the plate is completely recirculated back to the reactants via the lateral surfaces of the plate, thus supporting an excess enthalpy flame in the near-wake. The average output heat flux, defined as the total heat output through the lateral surfaces of the plate divided by the characteristic length associated with the temperature variation along the plate, is identified as an appropriate measure to characterize the heat recirculation efficiency.

Interplay of confinement and density on the heat transfer characteristics of nanoscale-confined gas

The effect of changing the Knudsen number on the thermal properties of static argon gas within nanoscale confinement is investigated by three-dimensional molecular dynamics simulations. Utilizing thermalized channel walls, it is observed that regardless of the channel height and the gas density, the wall force field affects the density and temperature distributions within approximately 1 nm from each channel wall. As the gas density is increased for constant channel height, the relative effect of the wall force field on the motion of argon gas atoms and, consequently, the maximum normalized gas density near the walls is decreased. Therefore, for the same Knudsen number, the temperature jump for this case is higher than what is observed for the case in which the channel height changes at a constant gas density. The normalized effective thermal conductivity of the argon gas based on the heat flux that is obtained by implementation of the Irving–Kirkwood method reveals that the two cases give the same normalized effective thermal conductivity. For the constant density case, the total thermal resistance increases as the Knudsen number decreases while for the constant height case, it reduces considerably. Meanwhile, it is observed that regardless of the method used to change the Knudsen number, a considerable portion of the total thermal resistance refers to interfacial and wall force field thermal resistance even for near micrometer-sized channels. It is shown that while the local thermal conductivity in the near-wall region strongly depends on the gas density, the wall force field leads to a reduced local thermal conductivity as compared to the bulk region.

Bifurcation parameters of a reflected shock wave in cylindrical channels of different roughnesses

To investigate the effect of bifurcation on the induction time in cylindrical shock tubes used for chemical kinetic experiments, one should know the parameters of the bifurcation structure of a reflected shock wave. The dynamics and parameters of the shock wave bifurcation, which are caused by reflected shock wave–boundary layer interactions, are studied experimentally in argon, in air, and in a hydrogen–nitrogen mixture for Mach numbers $$M =$$ 1.3–3.5 in a 76-mm-diameter shock tube without any ramp. Measurements were taken at a constant gas density behind the reflected shock wave. Over a wide range of experimental conditions, we studied the axial projection of the oblique shock wave and the pressure distribution in the vicinity of the triple Mach configuration at 50, 150, and 250 mm from the endwall, using side-wall schlieren and pressure measurements. Experiments on a polished shock tube and a shock tube with a surface roughness of 20 $${\upmu }$$ m Ra were carried out. The surface roughness was used for initiating small-scale turbulence in the boundary layer behind the incident shock wave. The effect of small-scale turbulence on the homogenization of the transition zone from the laminar to turbulent boundary layer along the shock tube perimeter was assessed, assuming its influence on a subsequent stabilization of the bifurcation structure size versus incident shock wave Mach number, as well as local flow parameters behind the reflected shock wave. The influence of surface roughness on the bifurcation development and pressure fluctuations near the wall, as well as on the Mach number, at which the bifurcation first develops, was analyzed. It was found that even small additional surface roughness can lead to an overshoot in pressure growth by a factor of two, but it can stabilize the bifurcation structure along the shock tube perimeter.

Optimizing commensality of radio continuum and spectral line observations in the era of the SKA

The substantial decrease in star formation density from z=1 to the present day is curious given the relatively constant neutral gas density over the same epoch. Future radio astronomy facilities, including the SKA and pathfinder telescopes, will provide pioneering measures of both the gas content of galaxies and star formation activity over cosmological timescales. Here we investigate the commensalities between neutral atomic gas (HI) and radio continuum observations, as well as the complementarity of the data products. We start with the proposed HI and continuum surveys to be undertaken with the SKA precursor telescope MeerKAT, and building on this, explore optimal combinations of survey area coverage and depth of proposed HI and continuum surveys to be undertaken with the SKA1-MID instrument. Intelligent adjustment of these observational parameters results in a tiered strategy that minimises observation time while maximising the value of the dataset, both for HI and continuum science goals. We also find great complementarity between the HI and continuum datasets, with the spectral line HI data providing redshift measurements for gas-rich, star-forming galaxies with stellar masses Mstellar~10^9 Msun to z~0.3, a factor of three lower in stellar mass than would be feasible to reach with large optical spectroscopic campaigns.

The Influence of Ethanol Blending in Diesel fuel on the Spray and Spray Combustion Characteristics

The influence of ethanol blending in Diesel fuel on the spray and spray combustion characteristics was investigated by performing experiments in an optically accessible high-pressure / high-temperature spray chamber under non-evaporating, evaporating and combusting conditions. Three fuels were investigated: (1) Diesel - a European Diesel based on the EN590 standard; (2) E10 - a blend of Diesel containing 10% ethanol and 2% emulsion additive; and (3) E20 - a blend of Diesel containing 20% ethanol and 2% emulsion additive. A constant gas density of 24.3 kg/m3 was maintained under non-evaporating (30 °C, 21.1 bar), evaporating (350 °C, 43.4 bar), low combustion temperature (550 °C, 57.3 bar) and high combustion temperature (600 °C, 60 bar) conditions. A single-hole injector with a nozzle diameter of 0.14 mm was used and injection pressure was held constant at 1350 bar. Various optical methods were used to characterize the non-combusting and combusting sprays. Despite the differences in the fuels' compositions, they did not differ significantly with respect to their liquid phase spray penetrations or cone angles under non-evaporating or evaporating conditions. However, under combusting conditions, reducing the ambient temperature increased the ignition delay and delayed the onset of soot formation for all fuels. Under equivalent combustion conditions, E10 and E20 had longer ignition and soot formation delays than Diesel. As the ethanol content of the fuel was increased from 0% to 20%, the lift-off length increased and the detectable soot luminescence decreased.

Engine combustion network: Influence of the gas properties on the spray penetration and spreading angle

In this work, three Engine Combustion Network (ECN) single-hole nozzles with the same nominal characteristics have been tested under a wide range of conditions measuring spray penetration and spreading angle. n-Dodecane has been injected in non-evaporative conditions at different injection pressures ranging from 50 to 150MPa and several levels of ambient densities from 7.6 to 22.8kg/m3. Nitrogen and Sulphur Hexafluoride (SF6) atmospheres have been explored and, in the first case, a temperature sweep from 300 to 550K at constant gas density has been executed. Mie scattering has been used as the optical technique by employing a fast camera, whereas image processing has been performed through a home-built Matlab code.Differences in spray penetration related to spray orifice diameter, spreading angle and start of injection transient have been found for the three injectors. Significant differences have been obtained when changing the ambient gas, whereas ambient temperature hardly affects the spray characteristics up to 400K. However, a reduction in penetration has been observed beyond this point, mainly due to the sensitivity limitation of the technique as fuel evaporation becomes important. The different behavior observed when injecting in different gases could be explained due to the incomplete momentum transfer between spray droplets and entrained gas, together with the fact that there is an important change in speed of sound for the different gases, which affects the initial stage of the injection.

Self-organized pattern formation in helium dielectric barrier discharge cryoplasmas

Self-organized patterns appear in many biological, chemical and physical systems, including electric discharges. Under certain conditions, self-organized patterns also form in plasmas generated below room temperature. These so-called cryoplasmas have also shown promise for low-damage materials processing; however, the underlying mechanisms and experimental conditions that lead to either uniform discharges or those containing self-organized patterns are still not understood completely. Here, we investigated the formation and dynamics of self-organized patterns in dielectric barrier cryoplasmas generated at plasma gas temperatures ranging from 264 down to 7 K at a constant gas density ρ = 5 × 1019 cm−3. The electrode gap was 0.15 mm and the cryoplasmas were generated at voltages between 0.8 and 1.5 kV, at frequencies ranging from 20 to 30 kHz. The discharges were characterized by time-resolved imaging, optical emission spectroscopy and current–voltage measurements. For temperatures down to 250 K, the discharges are uniform, whereas between 250 and about 140 K, self-organized, bright filamentary patterns form. Below that temperature, the discharge regime changes again to a uniform glow and for temperatures below 20 K, different types of discharges—uniform, but also self-organized dark solitons and bright stripe patterns—are observed. The cryoplasmas show current–voltage characteristics that are similar to atmospheric pressure glow discharges and the different types of uniform or self-organized discharges are suggested to be caused by the disappearance of impurities in the plasma as the temperature is lowered, and changes in the mobilities of ion species and surface charges.

LARGE-SCALE [OIII] AND [CII] DISTRIBUTIONS OF THE LARGE MAGELLANIC CLOUD WITH FIS-FTS

We present the results of far-infrared spectroscopic observations of the Large Magellanic Cloud (LMC) with FIS-FTS. We covered a large area across the LMC, including 30 Doradus (30 Dor) and N44 star-forming regions, by 191 pointings in total. As a result, we detect the [OIII] and [CII] line emission as well as far-infrared dust continuum emission throughout the LMC. We find that the [OIII] emission is widely distributed around 30 Dor. The observed size of the distribution is too large to be explained by massive stars in 30 Dor, which are assumed to be enshrouded by clouds with the constant gas density estimated from the [OIII] line intensities. Therefore the surrounding structure is likely to be highly clumpy. We also find a global correlation between the [OIII] and the far-infrared continuum emission, suggesting that the gas and dust are well mixed in the highly-ionized region where the dust survives in clumpy dense clouds shielded from energetic photons. Furthermore we find that the ratios of [CII]/CO are as high as 110,000 in 30 Dor, and 45,000 even on average, while they are typically 6,000 for star-forming regions in our Galaxy. The unusually high [CII]/CO is also consistent with the picture of clumpy small dense clouds.

Detailed analysis of particle launch velocities, size distributions and gas densities during normal explosions at Stromboli

Using high frame rate (33Hz) thermal video data we describe and parameterize the emission and ascent dynamics of a mixed plume of gas and particles emitted during a normal explosion at Stromboli (Aeolian Islands, Italy). Analysis of 34 events showed that 31 of them were characterized by a first phase characterized by an initial diffuse spray of relatively small (lapilli-sized) particles moving at high velocities (up to 213ms−1; average 66–82ms−1). This was followed, typically within 0.1s, by a burst comprising a mixture of ash and lapilli, but dominated by larger bomb-sized particles, moving at lower exit velocities of up to 129ms−1, but typically 46ms−1. We interpret these results as revealing initial emission of a previously unrecorded high velocity gas-jet phase, to which the lapilli are coupled. This is followed by emission of slower moving larger particles that are decoupled from the faster moving gas-phase. Diameters for particles carried by the gas phase are typically around 4cm, but can be up to 9cm, with the diameter of the particles carried by the gas jet (D) decreasing with increased density and velocity of the erupted gas cloud (ρgas and Ugas). Data for 101 particles identified as moving with the gas jet during 32 eruptions allow us to define a new relation, whereby Ugas=Uparticle+a [ρgasD]b. Here, Uparticle is the velocity of bombs whose motion is decoupled from that of the gas cloud, and a and b are two empirically-derived coefficients. This replaces the old relation, whereby Ugas=Uparticle+k D; a relation that requires a constant gas density for each eruption. This is an assumption that we show to be invalid, with gas density potentially varying between 0.04kgm−3 and 9kgm−3 for the 32 cases considered, so that k varies between 54m1/2s−1 and 828m1/2s−1, compared with the traditionally used constant of 150m1/2s−1.

Widely Extended [O III] 88μm Line Emission around the 30 Doradus Region Revealed with AKARI FIS-FTS

Abstract We present a distribution map of the far-infrared [O III] 88 $\mu$m line emission around the 30 Doradus (30 Dor) region in the Large Magellanic Cloud obtained with the Fourier Transform Spectrometer of the Far-Infrared Surveyor on board AKARI. The map reveals that the [O III] emission is widely distributed by more than 10$'$ around the super star cluster R 136, implying that the 30 Dor region is affluent with interstellar radiation field that is hard enough to ionize O$^{2+}$. The observed [O III] line intensities are as high as (1–2) $\times$ 10$^{-6}$ W m$^{-2}$ sr$^{-1}$ on the peripheral regions 4$'$–5$'$ away from the center of 30 Dor, which requires gas densities of 60–100 cm$^{-3}$. However, the observed size of the distribution of the [O III] emission is too large to be explained by massive stars in the 30 Dor region enshrouded by clouds with a constant gas density of 10$^{2}$ cm$^{-3}$. Therefore, the surrounding structure is likely to be highly clumpy. We also find a global correlation between the [O III] and the far-infrared continuum emission, suggesting that the gas and dust are well mixed in the highly ionized region where the dust survives in clumpy dense clouds shielded from energetic photons.

Study of the formation and propagation of a leader channel in air

Results are presented from calculations of the initial stage of leader channel formation in air. It is shown that the channel forms in two stages: one occurring at an essentially constant gas density on time scales much shorter than the characteristic gas-dynamic time and another in which gas-dynamic rarefaction of the channel becomes important and thermal-ionizational instability develops. The leader propagation velocity is largely determined by the time of channel contraction. The dependence of the leader velocity on the current and initial pressure is calculated. The results of calculations agree with experiment for relatively low leader currents (IL = 0.5–5 A) and for leader currents above tens of amperes. A comparative analysis of the leader propagation velocity in air at different initial pressures is given. In the current range IL = 0.5–30 A, the leader velocity depends weakly on the pressure; however, for currents as high as IL ≥ 50 A, it increases appreciably with pressure.

Edge-to-center plasma density ratio in high density plasma sources

The flux of positive ions leaving a classical low-temperature plasma discharge is proportional to the plasma density at the plasma–sheath edge, and the edge-to-center plasma density ratio, the so-called hl factor, normally depends only on the discharge size and the neutral gas pressure. The ion flux leaving the discharge is therefore linearly proportional to the central plasma density. The hl factor has been previously derived by solving the plasma transport equations over a large pressure range, with the assumption of constant neutral gas density within the discharge. Tonks and Langmuir derived the low pressure (collisionless) solution of this problem in 1929. More recent works have shown that the neutral gas density is no longer constant when the plasma pressure becomes comparable to the neutral gas pressure. In this paper, we solve the plasma transport equations in this new situation and we propose a new expression for the hl factor. It is shown that hl becomes a function of the central plasma density which implies that the ion flux leaving the discharge is no longer proportional to this density. This effect has to be included in particle and energy balance equations used in global models of high density plasma sources.

Anode plasma structure in a gas discharge with neutral density depleted by ionization

We consider the anode plasma structure in a gas discharge with density of neutral atoms (neutrals) depleted by strong ionization. We obtain analytical solutions of the quasi-neutrality equation for the potential distribution and a condition for the existence of anode plasma in the one-dimensional case for arbitrary potential dependences of the neutral depletion frequency and the electron density. We consider the special cases of a constant neutral depletion frequency, ionization by Maxwellian electrons, and ionization by an intense electron beam under the conditions of collisionless ion motion and Boltzmann thermal electron distribution. The solutions for the first two cases at zero depletion parameter, i.e., at constant gas density, match those obtained in [1] by a power series expansion. In the case of ionization by Maxwellian electrons, the formation of anode plasma at reasonable working-gas flow rates is shown to be possible only at a fairly high electron temperature (if, e.g., xenon is used as the working gas, then T e ≥ 5 eV). Steady-state solutions of the quasi-neutrality equation under ionization by an intense electron beam exist only if the ratio of the electron beam density to the maximum thermal electron density does not exceed a certain limiting value.

Modeling Polymer Electrolyte Fuel Cells with Large Density and Velocity Changes

A model fully coupling the flow, species transport, and electrochemical kinetics in polymer electrolyte fuel cells is presented to explore operation undergoing very large density and velocity variations. Comparisons are also made to a previous constant-flow model, which neglects the mass source/sink from the continuity equation and assumes constant gas density. Numerical results reveal large density (>50%) and velocity (>80%) variations occurring in the anode at anode stoichiometry of 1.2. In addition, the hydrogen concentration remained as high as the inlet owing to deceleration of the anode gas flow. Finally, the constant-flow model is accurate within 14% under common operating conditions, i.e., for anode stoichiometry ranging from 1.2 to 2.0. © 2005 The Electrochemical Society. All rights reserved.

Radial truncations in stellar discs in galaxies

We discuss the possible origin of the radial truncations in stellar discs, using measurements that we presented in an earlier paper (Kregel, van der Kruit & de Grijs, 2002). A tentative correlation is found with the de-projected face-on central surface brightness; lower surface brightness discs tend to have a smaller truncation radius in units of scalelength. This and our earlier finding that in smaller spirals the truncation tends to occur at a larger number of scalelengths are best reproduced when the truncation is caused by a constant gas density threshhold on star formation.

A dynamic model for power deposition in 3He lasers pumped by 3He(n,p) 3H reactions

The coupled variation of power density with gas density in a nuclear-pumped laser, which is excited by 3He(n,p) 3H reaction products, is considered. In the literature, volumetric excitation by reaction products of 3He(n,p) 3H is only considered for the case in which gas density is uniform and does not change during the pumping. In this work, a time-dependent model describing the coupled fluid dynamic and particle transport behaviour of the gas has been developed. In modelling charge particle transport behaviour, a previously reported energy deposition model for a constant gas density is extended for a variable gas density by taking into account variations in the particle range, macroscopic cross sections and neutron flux depending on density field of the gas. The coupled equations, which are obtained by using the power deposition density expression obtained for variable gas density in the acoustically filtered equations of motion of the gas, are solved numerically. Spatial and temporal variations of power deposition density and gas density during the pumping pulse are determined for various operating pressures ranging from 0.5 to 10 atm. In the calculations, the characteristics of I.T.U TRIGA Mark-II Reactor are used and it is assumed that laser tube is placed in the centre of the reactor core. Obtained results are presented and examined.

Study on ignition and combustion of dimethyl ether : air pre-mixture(CNG and Alternative Fuels, Oxygenated Fuels)

Over the last several years there has been much interest in dimethyl ether (DME) as an alternative fuel for diesel engines. DME combines advantages of a high cetane number with no soot combustion, which makes it highly suitable for compression ignition engines. On the other hand, a homogeneous charge compression ignition (HCCI) engine has been known to have high thermal efficiency, no soot and low NO_X emissions. DME for the HCCI engine has also good adaptability. But its ignition and combustion characteristics are not completely understood. The objective of this study is to investigate the ignition delay of DME as homogeneous mixtures with air by using a rapid compression machine. The investigated fuels are ethane and propane in addition to DME. When the air is used as the working gas, the maximum compressed gas temperature reaches 730K. This temperature can't realize the ignition of ethane or propane. Therefore, a mixture gas composition of 53% argon, 26% helium and 21% oxygen is used for realizing temperatures above 730K. Equivalence ratios are 0.2, 0.3 and 0.4. Ignition delays of each fuel are measured at the constant gas density with changing the compressed mixture temperature at the TDC. So far, the representative temperature at the ignition of fuel tended to use the mean gas temperature calculated by the perfect gas law using both of the measured cylinder pressure and the charged gas amount. But the temperature at the center of the combustion chamber measured by a thin Pt resistance wire thermometer having a diameter of 15 micron is used in this study. The experimental results under the constant piston displacement show that the ignition timings of ethane and propane are advanced with increasing in an equivalence ratio, on the contrary, the ignition timing of DME is late. Arrhenius plots of ignition delay on each fuel were drawn up from a series of combustion history data. The results show that ignition delay times of ethane and propane depend on both of the equivalence ratio and the gas temperature, whereas that of DME dose not depends on equivalence ratio, but only temperature. Also, the combustion histories of DME indicate two-stage combustion. The first combustion is known as the heat release by the low temperature oxidation reaction. The heat release amount during this reaction period is also independent of equivalence ratios. Furthermore, the numerical simulation of the DME combustion by means of CHEMKIN II was carried out. In the computation, detailed reaction scheme of DME proposed by Curran et al. was used in this study. Also, the computation results were compared with those from a rapid compression machine. Generally, a comparison of experiment results with computation results indicates a good agreement.